Pressure swing adsorption (PSA) is an adsorption process in which a gas mixture is contacted with a fixed adsorbent bed at high pressure to effectuate the removal of certain "adsorbate" constituents from the mixture. Although desorption can be accomplished by several means, the common characteristic of PSA adsorption cycles is that the adsorbent bed is regenerated by depressurizing and, in some applications, purging at low pressure. PSA enjoys commercial success in the areas of air separation, natural gas processing, solvent production, and various refinery applications. An account of the milestones in adiabatic adsorptive separations was given in the 1984 AIChE Symposium Series article, Twenty-Five years of Progress in "Adiabatic" Adsorption Processes, by Robert T. Cassidy and Ervine S. Holmes. PSA is an established unit operation with a myriad of applications.
The pressure swing adsorption process provides an efficient and economical means for separating a multi-component gas feed stream containing at least two gases having different adsorption characteristics. The adsorbable component can be an impurity that is removed from the less adsorbable component, which is taken off as a product, or the more adsorbable component can be the desired product, which is separated from the less adsorbable gas. For example, it may be desired to remove carbon monoxide and light hydrocarbons from a hydrogen-containing feed stream to produce a purified, i.e., 99+% hydrogen stream suitable for hydrocracking or other catalyst process where these impurities could adversely affect the catalyst or the reaction. On the other hand, it may be desired to recover more strongly adsorbable gases, such as ethylene from a feed stream to produce an ethylene-rich product.
In typical pressure swing adsorption, a multi-component gas is fed to at least one of a plurality of adsorbent beds at an elevated pressure effective to adsorb at least one component, i.e., the adsorbate fraction, while at least one other component passes through, i.e., the non-adsorbed fraction. At a defined time, the feed stream to the adsorbent bed is terminated and the adsorbent bed is depressurized by one or more cocurrent depressurization steps wherein pressure is reduced to a defined level. This permits the separated, less strongly adsorbed component or components remaining in the adsorption zone to be drawn off without significant concentration of the more strongly adsorbed components. The released gas typically is employed for pressure equalization and for subsequent purge steps. In the art and as used herein, "pressure equalization" means the connection of an adsorber at high pressure to an adsorber at low pressure until the pressures in the two adsorbers are equalized. The bed is thereafter countercurrently depressurized and often purged to desorb the more selectively adsorbed component of the feed stream from the adsorbent and to remove such gas from the feed end of the bed prior to the repressurization thereof to the adsorption pressure.
Such PSA processing is disclosed in U.S. Pat. No. 3,430,418 to Wagner, U.S. Pat. No. 3,564,816 to Batta and in U.S. Pat. No. 3,986,849 to Fuderer et at., wherein cycles based on the use of multi-bed systems are described in detail. As is generally known and described in these patents, the contents of which are incorporated herein by reference as if set out in full, the PSA process is generally carried out in a sequential processing cycle that includes each bed of the PSA system.
There are three basic mechanisms that are employed in PSA separations. They are rate selective, size-exclusive, and equilibrium selective. In rate selective separation, the primary driving force for the separation is the difference in the rates of adsorption, desorption, and diffusion of the various components in the gas to be separated. Thus, the flux of the adsorbable component into and out of the adsorbent controls the separation process steps within the PSA cycle. In size-exclusive separation, the primary mechanism for the separation of feed gas components is based on the size of the component molecules relative to the size of the adsorbent pores. The larger or bulkier molecules are simply excluded from the adsorbent while the smaller or narrower molecules are adsorbed, and the adsorbent pore dimensions determine which component is adsorbed and which is excluded. In equilibrium selective adsorption, it is the affinity for the adsorbent, or relative strength of adsorption of one component relative to another, which controls the separation. The less strongly adsorbed component becomes the non-adsorbable component and the most strongly adsorbed becomes the adsorbable component.
In size-exclusive and equilibrium selective driven fixed bed adsorption processes, mass transfer characteristics impact on efficiency and bed size. Due to mass transfer dynamics, an adsorbate concentration profile develops at the leading edge of the adsorption front as the front progresses through the bed. The term, "mass transfer zone," is familiar to those skilled in the art, and refers to the region that encompasses this profile, the leading edge of which contains a minimum concentration of adsorbate, and the trailing end of which contains the feed concentration of adsorbate. In order to obtain high purity, high pressure product, the adsorption step of a PSA cycle is usually discontinued before the leading edge of the mass transfer zone reaches the bed outlet. As a result, the adsorbent in the mass transfer zone is not fully loaded to its equilibrium capacity. For a discussion of mass transfer zone concepts and related adsorption topics, see: Lukchis, Adsorption Systems, Pan I-Design by Mass-Transfer-Zone Concept, Chemical Engineering, Jun. 11, 1973 at pp. 111-116; Lukchis, Adsorption Systems, Pan II-Equipment Design, Chemical Engineering, Jul. 9, 1973 at pp. 83-87; Lukchis, Adsorption Systems, Pan III-Adsorbent Regeneration, Chemical Engineering, Aug. 6, 1973 at pp. 83-90, the contents of which are herein incorporated by reference. Because the adsorbent in the mass transfer zone is not fully loaded and is, therefore, inefficient, inventors have sought to decrease the width of the mass transfer zone by improving mass transfer characteristics. By decreasing the width of the inefficient mass transfer zone, the overall adsorbent inventory requirement is decreased, and smaller beds result.
U.S. Pat. No. 5,051,115, issued to Leitgeb, et. at., discloses another method of improving PSA adsorption efficiency via mass transfer zone width minimization wherein an adsorber is repressurized in four steps. The first step comprises a pressure build-up phase with a weakly adsorbable gas; the second step continues pressure build-up with expansion gas from another, depressurizing adsorber; the third step effects further pressure build-up with high-pressure product gas from another, adsorbing adsorber; and the final step completes repressurization with feed. The reduction in mass transfer zone width is achieved by performing these repressurization steps within respectively specified pressure ranges, with the end result that yield is increased or adsorbent inventory is reduced.
It is known in the art that adsorption mass transfer characteristics are dependent on the velocity of the front and the rate at which adsorbate diffuses from the interparticle voids into the micropores of the adsorbent. The rate at which adsorbate diffuses, in turn, is highly dependent on temperature, concentration gradient, and particle size. Resistance to mass transfer also imposes other limitations on PSA designs, most prominently on desorption. Due to the rate limitation, the most important desorption design parameters are those that affect diffusion rates (such as temperature, particle size, driving force via concentration, and adsorbent diffusivity or permeability). The dynamics of the intermediate blowdown step are also mass transfer limited, but blowdown has traditionally received less attention because its effluent usually constitutes a small fraction of the overall product and because the rapid pressure changes complicate modelling and experimentation. A discussion of the effect of permeability and particle size is given by Lu, Intraparticle Diffusion/Convection Models for Pressurization and Blowdown of Adsorption Beds with Langmuir Isotherm, 27 Separation Science and Technology at 1857 (1992). The article teaches that the minimum blowdown time is set by the time required to reach near-equilibrium.
Rate-selective adsorption processes operate on the principal that preferential desorption occurs, not due to differences in adsorptive strength or by virtue of size exclusion, but by virtue of differences in component diffusivity and adsorption and desorption rates. Generally, molecules with smaller kinetic diameters will diffuse and adsorb more rapidly than those with large diameter. For example, nitrogen is separated from air using molecular sieve carbon (MSC). Although nitrogen is more strongly adsorbed on MSC at equilibrium, oxygen diffuses and adsorbs more rapidly. Therefore, oxygen is the preferentially adsorbed component despite its lower adsorptive strength. Examples of process improvements on the rate selective separation of nitrogen from air on MSC are U.S. Pat. No. 4,264,339, issued to Juntgen, and U.S. Pat. No. 4,376,640, issued to Vo. Juntgen's patent teaches that nitrogen purity, which was previously attainable only to 99.5, can be produced in 99.9% purity by adsorbing at continuously increasing pressure. Vo's patent describes a nitrogen from air separation improvement using MSC, whereby, during the latter portion of an adsorption step, high pressure effluent is used to repressurize a vessel that has just completed the desorption step.
U.S. Pat. No. 4,548,799, issued to Knoblauch et al., represents an improvement of the rate selective separation of nitrogen from oxygen-containing gas mixtures; in a two-bed system, nitrogen product purity is increased by discontinuing a pressure equalization step at between 0.3 to 0.7 seconds. The inventors teach that the oxygen content in the equalization stream increases with time and that, by discontinuing pressure equalization between 0.3 to 0.7 seconds, the equalization step ends before the increased oxygen content becomes counter-productive to product purity. It is not clear from the specification whether the source of the increased oxygen content is from oxygen desorption or whether it is simply oxygen from feed that previously occupied the interparticle voids in the lower regions of the bed.
U.S. Pat. No. 4,925,461, issued to Gemba et al., discloses a similar improvement of the rate selective separation of nitrogen from oxygen-containing gas mixtures. In multiple bed systems, a pressure equalization step is carried out by: connecting the inlets and outlets of each of the paired vessels; maintaining transfer flow between the inlets at 3 to 70 percent of the transfer flow between the outlets; discontinuing the step before the pressures in the paired vessels equalize; and feeding a back flow of product gas to the adsorber to be regenerated. The inventors teach that the beneficial effect of discontinuing the pressure equalization step early is due to the avoidance of rapid desorption of adsorbed components after pressure equalization. By these improvements, the nitrogen product, which had been previously limited to 99.9 percent purity, is thereby increased to 99.99 percent purity.
The growing importance of rate selective separation for air separation and for enhanced oil recovery has, in part, stimulated the development of advanced mathematical models that can predict mass transfer effects on the concentration profiles within an adsorbent bed. One approximation that is often used in the development of such models is the "frozen solid assumption," where it is assumed that no adsorption or desorption occurs during periods of rapid pressure change. The approximation was used in Yang, "Kinetic Separation by Pressure Swing Adsorption: Method of Characteristics Model," 36 AIChE Journal at 1229 (1990), an article that discusses a rate selective PSA separation. However, in Gas Separation by Adsorption Processes, Butterworth, Boston, 1987, Yang pointed out that the frozen solid assumption can lead to error when equilibrium conditions are nearly reached within the blowdown step. Such would often be the case in size-exclusive and equilibrium selective PSA separation processes.
In rapid pressure swing adsorption (RPSA) processes, small beds containing fine particle adsorbent are cycled rapidly to provide greater adsorbent productivity and a continuous product effluent. The basic cycle consists of a feed step and an exhaust step; the separation is driven by differences in adsorptive strength (equilibrium selective) or by size exclusion (size exclusive selective); and its advantages include smaller equipment size and the ability to operate with a single bed. U.S. Pat. No. 4,194,892, issued to Jones, et at., improved the cycle, which previously consisted of equal feed and exhaust times, by decreasing the feed time/(feed time plus exhaust time) ratio and incorporating a flow-suspension step between the feed and exhaust steps. In addition, smaller particle size was recognized as an advantage because smaller particles would improve mass transfer during the exhaust step. As illustrated by these improvements, RPSA benefits from an increase in mass transfer during the exhaust step. This is because the quantity of desorption product collected during exhaust is limited by mass transfer. Thus, in RPSA, high mass transfer rates improve the performance of the invention by increasing the approach to equilibrium during the RPSA depressurization/desorption step.
Generally, in size exclusive PSA applications, mass transfer rate has been a determining factor in the design of blowdown or pressure equalization steps. The separation of normal and isoparaffins using 5A molecular sieve is one such example. The separation is based on the size-exclusion of the branched isoparaffins from the adsorbent pores and the admission and adsorption of the normal paraffins, which are straight chain hydrocarbons. U.S. Pat. No. 5,146,037, issued to Zarchy et at., is an example of a size-exclusive PSA process to separate normal and isoparaffins. The PSA is combined with a catalytic process to produce an isomerized product. The adsorber is of sufficient capacity and the blowdown is controlled such that, as pressure is released during depressurization, the stoichiometric point of the mass transfer zone advances towards the end of the bed, but does not exit the bed. This description of front advancement implies that blowdown takes place at near-equilibrium. U.S. Pat. No. 4,608,061, issued to Volles et at., describes size-exclusive PSA process for the separation of normal and isobutane using 5A molecular sieve. The pressure equalization step in Volles et at. is counter-current, yet is carried out until pressure has fully equalized. U.S. Pat. No. 4,059,505 discloses a further modification to the process for separating normal and isoparaffins, whereby a mixture richer in normal paraffins than the feed is used as purge material during desorption. No reference is made to front advancement. However, the preferred time period for pressure reduction is stated as one to two minutes, during which period equilibrium would be approached.
One equilibrium selective PSA process is the separation of hydrogen from carbon oxides and/or light hydrocarbons. Instead of size-exclusion, the separation is based on differences in adsorptive strength. U.S. Pat. No. 3,564,816, issued to Batta, discloses a four-bed PSA process, whereby the adsorber is depressurized over a period of up to five minutes, using a sequence of (1) pressure equalization, (2) "provide purge," (3) another equalization, and (4) a blowdown step. During each successive step in the depressurization sequence, the impurity level rises, consistent with equilibrium desorption. U.S. Pat. No. 3,176,444, issued to Kiyonaga, teaches a PSA process whereby adsorption takes place at pressure and is discontinued prior to breakthrough so that, during cocurrent depressurization, although the stoichiometric point of the mass transfer zone advances towards the end of the bed, it does not exit the bed. Implicit in the specification is that blowdown takes place at near-equilibrium, so that the rate of desorption exceeds the rate of depressurization. U.S. Pat. No. 3,986,849, issued to Fuderer et al., teaches a process using additional beds to more efficiently pair equalization adsorbers at intermediate pressures, making the process more economical for high feed capacity. The inventors claim that the increased efficiency is due in pan to better disposition of the impurities caused by breakthrough during equalization. As described by Kiyonaga, breakthrough is attributed to front advancement during depressurization, consistent with equilibrium conditions. In Fuderer et al., total depressurization time lasts up to five minutes.
It is well known in the art that adsorbate loading is related to adsorbate partial pressure. This relationship is commonly expressed by isotherms, which are graphs of adsorbate loading versus the interparticle adsorbate partial pressure. A description of isotherms and methods for their generation are given in Breck, Zeolite Molecular Sieves, John Wiley and Sons, New York, 1974, and is hereby incorporated by reference. Such isotherms show that adsorbate loading decreases with decreasing adsorbate partial pressure. Thus, the difference in adsorption and desorption pressure is the driving force behind PSA separations. However, partially because of the transient nature of the blowdown step and the relatively small contribution of the blowdown effluent to the total desorption product, PSA art has not emphasized the control of compositional changes that occur during the blowdown step, except in terms of the disposition of front advancement.
Although equilibrium depressurization has been practiced in the prior art, and has been engineered to provide benefits such as the type described by Fuderer, U.S. Pat. No. 4,726,816, no one has of yet reported observation of a non-equilibrium depressurization in size-exclusive and equilibrium selective PSA applications. Furthermore, no one has suggested the intentional promotion of a disequilibrium state in size-exclusive and equilibrium selective PSA separations as a means of enhancing performance. Accordingly, inventors continue to seek new ways to improve PSA processes.
The objective of this invention is to provide a way to decrease the bed size or, alternatively, to increase the feed capacity of conventional size-exclusive and equilibrium selective PSA processes. It is a further objective of this invention to achieve these objectives at little or no added capital or utility cost.